Field of the Invention
The present invention relates to an image pickup apparatus having an electronic viewfinder function, such as a digital still camera and a digital video camera, a method of controlling the same, and a storage medium.
Description of the Related Art
Image pickup apparatuses, such as a digital still camera, include one having an electronic viewfinder function (EVF function) for displaying an image (moving image) of an object on an image display section of an apparatus body before photographing a still image. As a general operation of the electronic viewfinder function, a user checks a layout of an image to be photographed on the image display section, and when the user instructs the image pickup apparatus to photograph a still image e.g. by fully pressing a release button, the in-focus position is adjusted by an auto focus function, and then a still image is photographed and recorded.
In the process described above, a moving image displayed on the image display section is used for checking the image, and hence the frame rate in EVF processing is desired to be set to a relatively low rate, such as 30 fps, so as to reduce processing load. On the other hand, in performing auto-focus processing, it is necessary to more quickly determine the in-focus position to improve responsiveness of a series of processing operations from receiving a user's instruction to photographing a still image. Therefore, the frame rate in performing auto-focus processing is desired to be set to a higher rate, such as 240 fps.
Therefore, in general, a moving image of an object is displayed on the image display section at a low frame rate before photographing a still image, and when an instruction for photographing the still image is provided by a user's operation, the frame rate is changed to a high frame rate to perform auto-focus processing. However, in this case, when changing the frame rate, unnecessary waiting time is generated, which may cause undesired reduction of operability.
As a method of reducing such unnecessary waiting time, there has been proposed a technique that improves operability of a series of processing operations from receiving a photographing instruction by a user's operation to starting photographing of a still image, by simultaneously reading images from the image pickup device at a plurality of frame rates (see Japanese Patent Laid-Open Publication No. 2013-55589).
Further, there has been proposed an image pickup apparatus including a console section that outputs a temporary stop signal and a resume signal for temporarily stopping and resuming image pickup by an image pickup section, and a controller that controls image pickup by the image pickup section based on the temporary stop signal and the resume signal (see Japanese Patent Laid-Open Publication No. 2009-171004). In this proposal, image pickup performed by the image pickup section is temporarily stopped by a user's operation, whereby it is possible to perform photographing without recording an unnecessary object image.
However, in Japanese Patent Laid-Open Publication No. 2013-55589, during display of an object image on the image display section before photographing a still image, images are always simultaneously read out at the plurality of frame rates, which increases power consumption. Further, if images are simultaneously read out at the plurality of frame rates by the operation of pressing the release button, unnecessary waiting time is generated in changing the control of the image pickup device, and time lag caused by the waiting time reduces the operability of the camera.
On the other hand, in Japanese Patent Laid-Open Publication No. 2009-171004, image pickup by the image pickup section is temporarily stopped and resumed according to instructions provided by a user's operation, and hence it is impossible to determine based on object information whether or not a temporary stop period is required.
Further, conventionally, in the image pickup apparatus, the position information of an object for use in auto-focus control is obtained based on an image signal output from the image pickup device. Alternatively, the position information of an object is obtained based on a phase difference in an image indicated by an optical signal directly input to a dedicated detection device. In the case where the position information is obtained based on the image signal, the dedicated detection device can be dispensed with, and hence it is possible to reduce the size of the image pickup apparatus.
FIG. 7 is a timing diagram useful in explaining timing of an auto-focus image pickup operation (AF evaluation image pickup) of a conventional image pickup apparatus during live view.
In the conventional image pickup apparatus, the image pickup timing is defined by a vertical driving pulse (VD). When an AF control signal is turned on, an AF-evaluation image (image used for AF evaluation) is picked up by the image pickup apparatus according to the vertical driving pulse after a live view image pickup period. Then, when the AF control signal is turned off, the image pickup apparatus enters the live view image pickup period again.
As described above, the live view image pickup period during which a live view image is obtained and the AF operation period in which an AF evaluation image is obtained serially exist along the time axis, and hence the live view image and the AF evaluation image cannot be simultaneously picked up.
Therefore, the AF evaluation image is picked up during the AF operation period between the live view periods (live view frames), as shown in FIG. 7, and hence a time lag is inevitably generated between the live view image pickup and the AF evaluation image pickup.
Moreover, although the live view image display is performed even when the AF evaluation image is picked up, in this case, the live view image display is performed based on the AF evaluation image. As shown in FIG. 7, when the AF evaluation image is picked up, the frame rate is set to a higher rate than in the live view image pickup period, and hence a thinning ratio for reading by the image pickup device is increased, which inevitably degrades image quality. Therefore, it is difficult to avoid uncomfortable feeling due to degradation and fluctuation of image quality of the live view image.
To overcome this problem, there has been proposed an image pickup apparatus that is provided with focus detection pixels, separately from pixels for image pickup signals, in a pixel section of the image pickup device. As an example of the method of AF operation using the focus detection pixels, there has been known a so-called phase difference detection auto-focus (phase difference AF) method, as distinct from a contrast detection focus (contrast AF) method in which an in-focus position is detected based on an object image obtained from normal pixels (i.e. the pixels for image pickup signals).
In the contrast AF method, by paying attention to signals output from the image pickup device, particularly, high-frequency components (contrast information), a position of a photographic lens at which the evaluation value (contrast evaluation value) is largest is set to the in-focus position. In the contrast AF method, the contrast evaluation value is obtained while moving the photographic lens by a minute amount. Then, in the contrast AF control, it is necessary to drive the photographic lens until the position of the photographic lens is known at which the contrast evaluation value is largest.
On the other hand, in the phase difference AF method, a light flux passing through an emission pupil of the photographic lens is divided into two light fluxes, and the divided two fluxes are received by a pair of focus detection sensors (i.e. focus detection pixels), respectively. Then, a deviation amount between the image signals output in response to the light reception, i.e. a relative positional deviation amount in a direction of dividing the light flux is detected to thereby directly determine a deviation amount in a direction of focusing the photographic lens.
Therefore, in the phase difference AF method, by once performing an operation for accumulating electric charges using the focus detection sensors, it is possible to determine an amount of focus deviation and a direction thereof, whereby it is possible to perform the focus adjustment operation at a high speed.
To achieve both of high image quality of the live view image and high accuracy in focus detection, there has been proposed a technique of controlling an image pickup apparatus including an image pickup device with focus detection pixels in discrete arrangement, in which a manner of driving of the image pickup device is changed (see Japanese Patent Laid-Open Publication No. 2010-181751). In this technique, the image pickup apparatus has a first thinning read-out mode and a second thinning read-out mode which are different in at least one of a thinning ratio and a thinning phase, and selects the first thinning read-out mode or the second thinning read-out mode according to the state of the image pickup apparatus.
However, in the image pickup apparatus described in Japanese Patent Laid-Open Publication No. 2010-181751, when reading out the pixels, it is necessary to handle the focus detection pixels as defective pixels on the photographed image. For this reason, taking into account image quality of e.g. a still image using all pixels, it is difficult to arrange the focus detection pixels at a high density in all areas of the image pickup device.
Therefore, it is difficult to perform AF control with high accuracy by covering a sufficient angle-of-view area only with the focus detection pixels, and to improve the AF control performance, it is necessary to use the contrast AF method using the image signals in combination with the phase difference AF method using the focus detection pixels. In this case, in the contrast AF method, a time lag is generated due to the above-mentioned change of the manner of driving, and further, when degradation of image quality of the live view image is taken into account, the use of the phase difference AF method and the contrast AF method in combination makes it difficult to prevent a time lag from being generated, and image quality from being degraded.
In addition, if the use of the phase difference AF method and the contrast AF method in combination increases loads on both of the image pickup device driving and the image data processing circuit. Further, the use of the phase difference AF method and the contrast AF method in combination inevitably increases power consumption, so that electric power of a power source, such as a battery, is consumed in a short time period.